In telephony, an echo is a reflection of the voice signal. It is a delayed copy of the original. An example scenario is illustrated in FIG. 1a, which shows a signal being captured by a far-end microphone and output by a near-end loudspeaker. The echo is a consequence of acoustic coupling between the near-end loudspeaker and a near-end microphone; the near-end microphone captures the signal originating from its own loudspeaker in addition to the voice of the near-end speaker and any background noise. The result is an echo at the far-end loudspeaker. Echo cancellation is an important feature of telephony. Hands-free devices and teleconferencing, in particular, require echo cancellation that can adapt to environments having a wide range of acoustic characteristics.
Echo cancellers typically synthesise an estimate of the echo from the far-end voice signal. The estimated echo is then subtracted from the near-end microphone signal. This technique requires adaptive signal processing to generate a signal accurate enough to cancel the echo effectively. An adaptive filter is often used to model the environment's acoustic impulse response. Adaptation of this filter is usually frozen during near-end speech because otherwise the filter tries to adapt to the near-end speech, which causes its coefficients to diverge. Near-end speech (or double-talk) detectors are generally not sensitive to echo path changes, however. Consequently the adaptive filter may be frozen just when it needs to adapt at a faster rate.
There is a need for a practical system that is sensitive to changes in the microphone signal, so that an adaptive filter can be appropriately controlled.